The inquiry is slated to begin May 23, but that date could be pushed back if the public-sector union succeeds with its motion.

The public inquiry, which was ordered by the province in 2006, aims to examine how Manitoba's Child and Family Services officials failed to protect Phoenix and why her death went undiscovered for months.

Garth Smorang, a Winnipeg lawyer representing the MGEU, said the union is not trying to sidestep its responsibility in the little girl's death.

The union is simply questioning whether the inquiry is legally valid to begin with, he said.

"The scope of the jurisdiction given to the commissioner may exceed what is allowed under the Manitoba Evidence Act, and therefore beyond the scope of an inquiry," Smorang said Monday.

Scope too broad, lawyer says

Phoenix had spent much of her young life in foster care before her mother, Samantha Kematch, regained custody of her in 2004.

Phoenix was killed on the Fisher River First Nation in June 2005, but it wasn't until March 2006 that her body was found wrapped in plastic and in an unmarked shallow grave.

The little girl died after neglect and repeated abuse by Kematch and her common-law husband, Karl McKay, both of whom were convicted of first-degree murder in 2008.

Smorang said the first phase of the public inquiry, which will discuss how Phoenix's death went undiscovered for nine months, has a scope that falls outside the jurisdiction of the Manitoba Evidence Act, the legislation under which the inquiry was called.

The scope of that part of the inquiry is too broad, Smorang argued, as it is covered by the Child Fatalities Act and the Child and Family Services Act.

Chiefs call move a stalling tactic

In a statement, Family Services Minister Jennifer Howard said the provincial government will be in court on Thursday to defend the inquiry "because we believe in accountability."

Inquiry commissioner Ted Hughes "disagrees with the MGEU's position and will defend this inquiry as well," Howard added.

Aboriginal leaders called the MGEU's move a stalling tactic that could endanger the lives of children.

"There's always discussions about what kind of accountability is there for First Nations people," Grand Chief Derek Nepinak of the Assembly of Manitoba Chiefs said Monday.

"Now what we're faced with here is … laws and rules that are being implemented so that nobody has to be accountable for their actions when dealing with our children."

Lawyers for the MGEU are scheduled to appear before the Manitoba Court of Appeal on Thursday morning to set a hearing date.

Smorang said the court can deal with the MGEU's motion quickly.

Sinclair died during a period of devolution in which aboriginal child-welfare agencies were taking over responsibility for aboriginal children in care.

Leaders with the Southern Chiefs Organization say they hope the Sinclair inquiry can be put back on schedule in order to address practical issues, such as the reunification of aboriginal children in care with their biological parents.

SCO Grand Chief Morris Swan-Shannacappo said leaders are facing pressure from people who encounter hurdles in their bids to get their children back.